The present invention relates to thermal printers and more particularly to a thermal printer donor media element having a single track code including multiple sequential data fields identifying parameters for enabling the printing process such as, but not limited to, media type, and color and location of successive color patches, and apparatus for detecting and reading the code.
Today's thermal printers are designed with two or more different color LEDs (typically two LEDs wherein one LED is blue and the second is green or three LEDs wherein one LED is blue, the second is green, and the third is red) and two or more sensors. Mechanically, the donor material is between the LEDs and sensors. The donor media roll contains three or more sections of different media color type in a specific serial order, repeating each pattern throughout the entire roll. Typically, three areas of color specific donor material is required for a photographic quality thermal hard copy print. For a typical donor, that sequence of color specific donor material may be yellow, magenta, and cyan. Other donor materials may be composed of a base sequence of four color specific donor areas: yellow, magenta, cyan, and black. Still other donor materials may additionally include a clear laminate in the sequence for coating the print with a photographlike finish. The particular sequence of donor material (whatever that may be) is repeated in a serial fashion to complete the entire roll of donor material.
For identifying the color of a particular donor material section, each LED simultaneously emits its specific wavelength spectra of light through the donor media section as the donor media is advanced through mechanical sprockets working the rollers of a roll of donor media. In real time, the sensors (typically one each for each LED) sense the media filtered light from each LED. The analog signal is either A/D (analog to digital converted) or compared to a reference and a digital high or low voltage is outputted and either of which are sampled in real time. With the digital data from each LED and using boolean algebra logic, the type (color) of the section of media (immediately between the LEDs and sensors is determined. This information is used to define the color type of media material and sense the leading edge of the next section of donor media material both of which are used by the printer CPU (central processing unit) to perform the correct printing process of which a typical process is described in the next paragraph.
In producing a thermal hard copy output, donor material (usually the least thermally active color) is positioned over the thermal paper. Mechanical rollers, edge and color sensors are used to recognize and position the desired donor material color over the thermal output paper. A thermal head, in which pixels (typically 300 per inch) are arranged in a linear fashion, is positioned at the edge of the thermal output paper, the donor material being located between the thermal head and the thermal output paper. Digitized control data is then applied to each pixel simultaneously (usually pulse modulated) such that a row or line of one color is printed onto the thermal paper. Through stepper motors and mechanics, and control logic, either the thermal print head or thermal output paper is advanced one line or row and the thermal transfer process is repeated for that row. This whole sequence is repeated until one color is thermally transferred onto one full sheet of desired thermal output paper. The thermal paper is projected, donor material is advanced to the next color area, and the thermal output paper is re-inserted. The LED sensor system is used to determine the type of media and the leading edge of the new media section. The entire process is repeated until the next color in the sequence is transferred onto the thermal output paper. This process is repeated again with donor material advancing to the next color area until all colors of the donor material are transferred or thermally printed onto the thermal output.
With two or more LEDs and sensors, the above described system has cost disadvantages as well potential robustness issues. Robustness can be low for a two LED system with today's state of the art in LED emission and detection. As one improves robustness with a three LED system, cost is further disadvantaged. As an additional problem, it has been found that, in the event the printer is unable to recognize a color, the printer may go to an indeterminate state, such that the printing operation is interrupted or halted, resulting in an incomplete output.
It is also known to provide machine readable indicia such as bar codes and the like in tracks of code or marks on the donor media itself, or on a cartridge, case or spool containing the donor media, which respective tracks typically include information relating to various parameters such as donor type, identification and location of color segments or patches, as well as metering or timing marks to enable more accurately reading data streams that are detected or sensed at varying speeds. Reference, for example, U.S. Pat. No. 5,009,531 entitled "Color Ink Ribbon and Printer Using this Ribbon" issued Apr. 23, 1991 in the name of Seiji Koike which discloses the use of a track of bar coded color recognition marks, a track of speed detection marks, and a compensation coefficient on a body of the ink ribbon for determining identity of color portions on the ink ribbon. Reference also U.S. Pat. No. 5,786,841 entitled "Single Track of Metering Marks on Thermal Printer Media" issued Jul. 28, 1998, in the name of Mark A. Bobb et al. which discloses the use of a single track of differently spaced groups of metering marks on thermal printer donor media for identifying the color and location of the beginning of color patches on the media.
However, the Koike ribbon requires multiple sensors for detecting the various marks and compensation coefficients so as to suffer from the disadvantages discussed above, and Bobb et al. requires metering marks that are spaced sufficiently far apart so as to require substantial movement of the media for accurate detection of the groups for ascertaining the color and location of the beginning of the next color patch, so as to add to the printing time and increase the potential for error in color determination. Also, although the metering marks can be utilized for identifying color and location of color patches, additional means such as those discussed above are still required for accurately achieving desired color robustness. This is because in the thermal printing process, different types of donor media typically require different thermal energy levels for producing a particular robustness of a given color. Thus, some information, such as the media type, or a compensation coefficient or factor, must still be provided to enable determining the required energy level for the particular robustness.
Therefore, there is a need to provide a system for storage of data on thermal printer donor media including information enabling making accurate color and robustness determinations, which data can be detected using simpler detection means, such as a single LED/sensor set, and which information can be accurately determined with minimal movement of the donor media and when less than all of the data is read.